Tropical peatland carbon storage linked to global latitudinal trends in peat recalcitrance The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Hodgkins, S.B., et al. "Tropical peatland carbon storage linked to global latitudinal trends in peat recalcitrance." Nature Communications 9,1 (2018): 3640. https://doi.org/10.1038/ s41467-018-06050-2 © 2018 Author(s) As Published 10.1038/S41467-018-06050-2 Publisher Springer Science and Business Media LLC Version Final published version Citable link https://hdl.handle.net/1721.1/125774 Terms of Use Creative Commons Attribution 4.0 International license Detailed Terms https://creativecommons.org/licenses/by/4.0/ ARTICLE DOI: 10.1038/s41467-018-06050-2 OPEN Tropical peatland carbon storage linked to global latitudinal trends in peat recalcitrance Suzanne B. Hodgkins 1,2, Curtis J. Richardson3, René Dommain4,5, Hongjun Wang 3, Paul H. Glaser6, Brittany Verbeke7, B. Rose Winkler7, Alexander R. Cobb8, Virginia I. Rich2, Malak Missilmani9, Neal Flanagan3, Mengchi Ho3, Alison M. Hoyt10, Charles F. Harvey11, S. Rose Vining12, Moira A. Hough13, Tim R. Moore14, Pierre J. H. Richard15, Florentino B. De La Cruz 16, Joumana Toufaily9, Rasha Hamdan9, William T. Cooper1 & Jeffrey P. Chanton7 1234567890():,; Peatlands represent large terrestrial carbon banks. Given that most peat accumulates in boreal regions, where low temperatures and water saturation preserve organic matter, the existence of peat in (sub)tropical regions remains enigmatic. Here we examined peat and plant chemistry across a latitudinal transect from the Arctic to the tropics. Near-surface low- latitude peat has lower carbohydrate and greater aromatic content than near-surface high- latitude peat, creating a reduced oxidation state and resulting recalcitrance. This recalcitrance allows peat to persist in the (sub)tropics despite warm temperatures. Because we observed similar declines in carbohydrate content with depth in high-latitude peat, our data explain recent field-scale deep peat warming experiments in which catotelm (deeper) peat remained stable despite temperature increases up to 9 °C. We suggest that high-latitude deep peat reservoirs may be stabilized in the face of climate change by their ultimately lower carbo- hydrate and higher aromatic composition, similar to tropical peats. 1 Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306, USA. 2 Department of Microbiology, The Ohio State University, Columbus, OH 43210, USA. 3 Duke University Wetland Center, Nicholas School of the Environment, Durham, NC 27708, USA. 4 Institute of Earth and Environmental Science, University of Potsdam, 14476 Potsdam, Germany. 5 Department of Anthropology, Smithsonian Institution, National Museum of Natural History, Washington, DC 20013, USA. 6 Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, USA. 7 Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA. 8 Center for Environmental Sensing and Modeling, Singapore- MIT Alliance for Research and Technology, Singapore 138602, Singapore. 9 Laboratory of Materials, Catalysis, Environment and Analytical Methods (MCEMA-CHAMSI), EDST and Faculty of Sciences I, Lebanese University, Campus Rafic Hariri, Beirut, Lebanon. 10 Max Planck Institute for Biogeochemistry, 07701 Jena, Germany. 11 Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 12 Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ 85716, USA. 13 Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85716, USA. 14 Department of Geography, McGill University, Montreal, QC H3A 0B9, Canada. 15 Département de Géographie, Université de Montréal, Montréal, QC H2V 2B8, Canada. 16 Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, NC 27695, USA. These authors contributed equally: Curtis J. Richardson, René Dommain. Correspondence and requests for materials should be addressed to S.B.H. (email: [email protected]) or to J.P.C. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:3640 | DOI: 10.1038/s41467-018-06050-2 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-06050-2 eatlands are a major global carbon reservoir (528–600 Pg), are key to peat formation across all climates, other drivers differ with a significant portion of this carbon mass (10–30%) in between climatic zones and peat depths, with cold temperatures a P 1–4 tropical peatlands . Peat accumulation occurs when net key factor at high latitudes and more recalcitrant organic matter a primary productivity exceeds the rate of carbon loss via fires and key factor at low latitudes and deeper depths. decomposition, which is inhibited at high latitudes by anaerobic conditions5 and cold temperatures6. The existence of large peat deposits at low latitudes, where year-round warm temperatures Results and Discussion would be expected to drive higher microbial decomposition Differences in peat preservation mechanisms with latitude.In rates7,8, is thus surprising. Several hypotheses have been proposed this study, we have focused on two solid-phase organic matter to explain the accumulation of peat in these environments, such components that have been shown to drive peat decomposition: as higher primary productivity close to the equator9 that may carbohydrates that are the most labile solid-phase component20, allow faster litter deposition, as well as physical and chemical peat and aromatics that inhibit anaerobic decomposition14,23. These characteristics that may slow decomposition rates. For example, components produce distinct peaks in the FTIR spectra (Sup- peat in tropical peat swamp forests is largely composed of coarse plementary Fig. 1; Supplementary Table 1). Based on the tech- woody material from fallen trees, branches, and dead roots10. niques used to calibrate these FTIR peaks (see Methods; ref. 22; This material may be protected from decomposition by its low Supplementary Fig. 2), carbohydrates consist of acid-hydrolysable surface-area-to-volume ratio and high lignin content11,12, which polysaccharides, whereas aromatics consist of lignin and other has been hypothesized to severely limit its anaerobic decom- unsaturated acid-insoluble material such as tannins and humic position13,14. Low-latitude peat decomposition may also be slo- substances. While other components such as aliphatics have been wed by other chemical processes, including release of shown to correlate with peat humification20, these components decomposition-inhibiting phenolics from shrubs in unsaturated have not been identified as active in the humification process24 shrub peatlands15 and high organic matter recalcitrance following (unlike carbohydrates that are preferentially lost20 and aromatics initial rapid decay of plant litter16,17. These effects can be suffi- that can actively inhibit decomposition14,23), but most likely cient to preserve peat even in partially unsaturated conditions15. become concentrated as labile components degrade. However, their potential to preserve high-latitude peat as the Our results clearly show lower carbohydrate and greater climate warms and as woody species expand remains uncertain. aromatic content in temperate to tropical sites compared with Here we examined the role of peat and parent plant chemistry, Arctic and boreal sites (Fig. 2). Aliphatic content was slightly in particular the relative abundances of carbohydrates (i.e., higher in temperate to tropical sites, but this difference was much O-alkyl C or polysaccharides) and aromatics, in driving peat less pronounced (Supplementary Fig. 3). On average, surface peat formation and preservation along a latitudinal transect of major (<50 cm) north of 45°N had higher carbohydrate than aromatic peatland regions from the Arctic to the tropics (Table 1; Fig. 1). content, whereas surface peat south of 45°N had lower Relative abundances of carbohydrates and aromatics are indica- carbohydrate than aromatic content (Fig. 3a, b; Supplementary tors of organic matter reactivity, with lower carbohydrate and Fig. 4). Linear regression analysis (Fig. 3) of surface peat higher aromatic content indicating greater humification and/or carbohydrate and aromatic contents vs. latitude and mean annual recalcitrance18–21. In this study, we used a newly developed temperature (Supplementary Table 2) showed that these trends approach for Fourier transform infrared spectroscopy (FTIR) were significant. The overall highest aromatic concentration was analysis (see Methods), which is based on area-normalized peak found in the equatorial Mendaram peatland (Fig. 3). This result is heights calibrated to wet chemistry analyses in a set of standard consistent with previous FTIR and lignin phenol analyses at this materials22, to estimate carbohydrate and aromatic content in site11,12, which showed very high lignin content and smaller peat from high-latitude, mid-latitude, and low-latitude field sites. carbohydrate peaks than our northern sites. Because peat chemical composition is strongly affected by parent The latitudinal trends in carbohydrate and aromatic content vegetation in addition to humification18, we also analyzed selec- were also visible via principal components analysis (PCA) of the ted plant samples to distinguish the effects of humification